There was a Jewish school for rabbis.... and a very important, very influential center for Jewish thought and decision.... founded in Jamnia probably shortly after the Fall of the Temple in 70AD. It became very important.
It was universally held (until very recently) that (among many other things) it codified the Jewish Scriptures (essentially 39 books of the OT) and this was an official decision. This was pretty much universally held by historians to have happened in or around 90 AD. Jews taught this. Catholics taught this. Orthodox taught this;. Protestants taught this.
Well, quite recently... especially Catholics... have come to question whether such a meeting ever actually happened. Many believe that the school at Jamnia did hold that the Divine Scriptures are the 39... and that that view became common among Jews... but the new view is that it seems less likely there was some OFFICIAL, BINDING declaration or that the issue was authoritatively, definitively "settled" there and then. Indeed, some today doubt there ever was a Council of Jamnia. And some evidence suggests that the "canon" for Jews remained fluid perhaps until the middle ages. In other words, historians TODAY increasingly hold that for the Jews (just like for Christians) what IS and IS NOT canonical was a matter of slowly evolving TRADITION and not any official declaration of some authoritative group. Thus, Christians who insist that there was a CANON (of either OT or NT) books in the early centuries after Christ are probably wrong. The modern view is that for BOTH Jews and Christians (and thus both the OT and NT, not to mention Deuterocanonical works) this was a slow, evolutionary, imperfect, not always universal TRADITION, never "settled" by either religion: neither put anything "in" or took anything "out". Individual believers, together EVENTUALLY came to embrace what we see today. What is canonical, deuterocanonical and not canonical is, many now hold, something BELIEVERS determined although not in any authoritative way and perhaps not until the Middle Ages. For both Jews and Christians. And it's still not perfect.
We cannot know WHEN or HOW or WHY this happened since there was no Ruling Body making some Decision... and we have no way to know what moved Christians (or JEws) to do what they did... and no scientific surveys to show this. We only have this: Increasing, evolving evidence of a corpus of books seen as canonical (while others still remaining "in the mix" but not as fully canonical).
IMO, the new revisionist view probably needs more evidence.... but it does seem true the long held historical view of the Council and its Decision lacks evidence and has decreasing support. The "victim" of this is the view that there was a CANON - for Jews or Christians - ever official or formally declared. Or even universally embraced - for Jews or Christians. What is accepted today as "canonical" may not be the result of anything (no decision, no council) .... just believers SLOWLY embracing ever-evolving Tradition, perhaps not so solid until the early middle ages. Thus, the claim that the Early Church put "in" some books that some mysterious group took "out" is largely being repudiated today. No one and no thing can take "out" what none ever put "in." It's TRADITION...formed by ALL Christians (and Jews for the OT)... and it's not perfect. Some books have MORE embrace, some less, some no longer any at all.
Perhaps a spiritual perspective is that GOD lead HIS PEOPLE (as was promised), not working through any denomination or formal council, but simply among His people.... in what was a slow, evolving, growing consensus of belief... and perhaps rejoice that for perhaps 1500 years, ALL Christians it seems came to embrace 66 books (by our count)... with some others (never agreement on WHICH) viewed at DEUTEROcanonical.